Signal transmission speed in trunk line optical networks increases year by year, due to increase in traffic in communication lines. For this reason, there is a demand for increase in capacity of optical networks. Effective methods for increasing capacity of optical networks include a method of increasing a multilevel number in the modulation method, and a method of increasing the symbol rate, in addition to a coherent technique. To increase the symbol rate, it is desirable to reduce skew serving as shift of phase between an I (In-phase) component and a Q (quadrature) component of the signal.
For this reason, a known technique has a structure in which skew is changed in the device on the transmission side to transmit an optical signal, the quality of the received optical signal is measured in the device on the reception side, and the skew in the device on the transmission side is adjusted on the basis of the measured quality. The quality of the received optical signal is evaluated on the basis of, for example, the bit error rate of the received data. Because the quality of the optical signal received with the device on the reception side increases when the skew is small, the skew in the device on the transmission side is adjusted to maximize the quality of the optical signal in the device on the reception side. Conventional techniques are described in Japanese Laid-open Patent Publication No. 2013-207603 and Japanese Laid-open Patent Publication No. 2010-193204, for example.
The quality of the optical signal received with the device on the reception side is influenced by various factors as well as skew. For this reason, the quality of the reception signal may differ, even when the skew is the same. In addition, in the case where the quality of the reception signal is evaluated with the error rate after error correction, when the quality of the optical signal is equal to or higher than a predetermined level, the error rate becomes 0% (error free) and causes difficulty in specification of the peak of the quality of the reception signal. This causes difficulty in adjustment of the skew with high accuracy.